Polypeptide neurotoxins from sea anemone venoms bind in a voltage-dependent manner to Na channel receptor 3 and thereby reduce the rate of inactivation of this ion channel. These toxins display considerable selectivity of action upon Na channels in different animal phyla. The arthropod Na channel has a particulary high affinity for most sea anemone toxins. In this project we shall identify those chemical groups on the toxin's receptor binding domain which are important for interacting with and stimulating this polypeptide receptor. Elucidation of the special binding requirements of the vertebrate and arthropod forms of this receptor should eventually permit the design of selective drugs and insecticides which only act upon certain Na channels. We shall determine the amino acid sequences of four anemone neurotoxins which are highly selective for either arthropod or vertebrate Na channels. Toxin analogs will be prepared by chemical modification and peptide semisynthesis in order to elucidate how structure determines activity. Our investigation will focus upon the roles of acidic, basic, and hydroxyproline residues within the N-terminal sequence in determining toxin binding and efficacy upon vertebrate as well as arthropod site 3 receptors. Native and chemically-modified forms of anthopleurin B, one of the most active anemone toxins on vertebrate Na channels, will be used to investigate the binding requirements of the vertebrate site 3 receptor. We shall determine the influence of membrane potential upon anemone toxin binding to the lobster Na channel receptor. We shall further test the reported observation that the voltage-dependence of action of certain sea anemone toxins upon the sodium channel differs significantly. Using an affinity-label toxin derivative, we shall determine the polypeptide composition of an arthropod Na channel.